Interpretive Summary: Over 20 percent of the U.S> land area and approximately 50 percent of the world’s arable lands are acidic (pH - 5). On these acid soils, aluminum (Al) toxicity is the primary factor limiting crop production. There is considerable interest and research efforts in identifying genes that provide tolerance to Al toxicity. One of the major physiological mechanisms for crop plants to cope with Al stress involves the efflux of organic acids (OA) from roots. In this study, we found that the region that controls gene expression, the promoter, plays a key role in Al tolerance. Using a gene technique, we found that the expression of the gene associated with citrate a particular OA efflux conferred the highest level of Al tolerance at the lowest carbon cost. Our results indicate that in biotechnological approaches may be used to improve Al-tolerance of crops.

Technical Abstract:
In Arabidopsis, aluminum (Al)-activated AtALMT1-mediated root malate exudation plays a major role in Al tolerance, while Al-activated AtMATE-mediated citrate exudation plays a much smaller role. In this study, we demonstrate that the levels of Al-activated root organic acid exudation are closely correlated with the transcriptional levels of the organic acid transporter genes, AtALMT1 and AtMATE. We found here that the AtALMT1 promoter is stronger than the AtMATE promoter. From this result, in conjunction with the fact that citrate is a much more effective Al chelator than malate, we adopted a promoter swap strategy to test if the expression of the AtMATE gene driven by the strong AtALMT1 promoter will result in increased Al tolerance in Arabidopsis. Our results indicated that the AtALMT1:AtMATE promoter swap not only increased Al tolerance of the transgenic plants, but also enhanced carbon usage efficiency for Al tolerance.